The chief scientific adviser has become a government spin doctor

I report this with sadness: Sir David King has lost his bottle. Until a few weeks ago, the chief scientific adviser looked to me like one of the few brave souls in the British government. In an article in Science at the beginning of last year, he argued that "climate change is the most severe problem that we are facing today - more serious even than the threat of terrorism" and criticised the Bush administration for "failing to take up the challenge". In response, he was viciously attacked by the Exxon-sponsored climate change denier Myron Ebell. Being viciously attacked by Ebell is something to which all self-respecting scientists should aspire.

Last month he was attacked again, and this time he deserved it. At a meeting of climate change specialists, King announced that a "reasonable" target for stabilising carbon dioxide in the atmosphere was 550 parts of the gas per million parts of air. It would be "politically unrealistic", he said, to demand anything lower.

Simon Retallack, from the Institute for Public Policy Research, reminded King what his job was. As chief scientist, his duty is not to represent political reality - there are plenty of advisers schooled in that art - but to represent scientific reality. Retallack's own work, based on the latest science, shows that at 550 parts per million the chances of preventing more than two degrees of global warming are just 10%-20%. To raise them to 80%, carbon concentrations will have to be stabilised at 400 parts. Two degrees is the point beyond which most climate scientists predict catastrophe: several key ecosystems are likely to flip into runaway feedback; the biosphere becomes a net source of carbon; global food production is clobbered, and 2 billion people face the risk of drought. All very reasonable, I'm sure.

King replied that if he recommended a lower limit, he would lose credibility with the government. As far as I was concerned, his credibility had just disappeared without trace. By shielding his masters from uncomfortable realities, he is failing in his duties as both scientist and adviser. Anyone who has studied the BSE crisis knows how dangerous the cowardice of scientific counsellors can be.

As if to prove that his nerve has gone, on Friday King made his clearest statement yet that he sees nuclear power as the answer to climate change. With the right carbon taxes, he said, nuclear power would become cheaper than coal. "It's important we do take the public with us on the environmental debate," he said. "That is why I'm trying to sell it." He may have political reasons for "trying to sell" new nuclear power stations - at the Labour party conference Tony Blair said he wants to re-examine the nuclear option - but King would, I suspect, have as much trouble identifying a scientific case as he had at the meeting last month. The figures leave him stranded.

Let us forget, for the moment, that nuclear power spreads radioactive pollution, presents a target for terrorists and leaves us with waste that no government wants to handle. Like King, I believe that while all these problems are grave, they are not as grave as climate change. Let us concentrate on value for money.

It seems clear that new nuclear power stations will not be built unless the government supports them. A recent review by the economics consultancy Oxera shows that even if you exclude the cost of insurance and include the benefits of emissions trading (which attaches a price to carbon dioxide), "a programme of public assistance ... would be needed to boost predicted [rates of return] to a level that is acceptable to private investors". The consultants suggested that £1.6bn of grants might be enough to tip the balance in favour of a new nuclear programme.

The first "even if" is a big one. Private insurers will not cover the risk. Three international conventions limit investors' liability and oblige governments to pick up the bill on their behalf. According to a report commissioned by the European parliament, the costs of a large-scale nuclear accident range from €83bn to €5.5 trillion. They would have to be met by us.

But let us also forget the costs of insurance. If the public sector (or for that matter, given that funds are limited, the private sector) invests in nuclear power, is this the best use to which the money can be put? This is the question addressed in a new paper by the physicist Amory Lovins.

He begins by examining the terms of reference used by people like King, who compare nuclear power "only with a central power plant burning coal or natural gas". If the costs of construction come down, and if the government offers big enough subsidies and makes carbon emissions sufficiently expensive, Lovins says, nuclear power might be able to compete with coal. "But those central thermal power plants are the wrong competitors. None of them can compete with windpower ... let alone with two far cheaper resources: cogeneration of heat and power, and efficient use of electricity."

Ten cents of investment, he shows, will buy either 1 kilowatt-hour of nuclear electricity; 1.2-1.7 of windpower; 2.2-6.5 of small-scale cogeneration; or up to 10 of energy efficiency. "Its higher cost than competitors, per unit of net CO2 displaced, means that every dollar invested in nuclear expansion will worsen climate change by buying less solution per dollar." And, because nuclear power stations take so long to build, it would be spent later. "Expanding nuclear power would both reduce and retard the desired decrease in CO2 emissions."

Already, the market is voting with its wallet. "In 2004 alone," Lovins notes, "Spain and Germany each added as much wind capacity - 2bn watts - as nuclear power is adding worldwide in each year of this decade." Though the nuclear industry in the US has guzzled 33 times as much government money as wind and has "enjoyed a regulatory system of its own design for a quarter-century", it hasn't fulfilled a single new order from the electricity companies since 1973. And, unlike nuclear power stations, wind, cogeneration and energy efficiency will all become much cheaper.

It's certainly a good idea, as people like King recommend, to have a "diversified energy portfolio". But, as Lovins points out, "this does not mean ... that every option merits a place in the portfolio purely for the sake of diversity, any more than a financial portfolio should include bad investments just because they're on the market". Building new nuclear power stations in Britain would be a political decision, not a scientific one.

So what has happened to the man who once bravely did battle with the new Inquisition? A memo sent by Blair's private secretary, Ivan Rogers, a month after King's article was published in Science, instructed him to stop criticising the Bush administration on the grounds that it "does not help us achieve our wider policy aims". Mock interviews King conducted with his political minders, which were found by a journalist on a disk dropped by his press secretary, show him learning to recite the government's line. Could he have had his arm twisted over the nuclear issue too?

I hope not, and I hope he can produce some robust figures to support his contentions. But I fear that the government's chief scientist is mutating into its chief spin doctor.